1. Field of the Invention
The present invention generally relates to protecting composite structures from lightning strikes, and more particularly, to an appliqué for protecting composite aircraft from lightning strikes.
2. Background Description
Since aluminum and other metals are highly conductive, a transient charge from a lightning striking a metal body discharges into the metal body with current from the discharge being distributed relatively evenly over the body. So, a typical lightning strike to a metal aircraft causes no or only minor damage to aircraft components. However, carbon fiber composites generally have a higher strength-to-weight ratio than aluminum, and so, are increasingly replacing aluminum structural components. Unfortunately, typical state of the art composites, such as Carbon Fiber Reinforced Plastic (CFRP), are approximately 2000 times more resistive than aluminum.
So, a lightning strike that may have little or no effect on an aluminum structure may affect unprotected CFRP components. For adequate lightning protection for a composite wing structure, the exterior CFRP structure must withstand not only the initial lightning strike, but also at least one hundred kiloamperes (100 kA) of discharge current without adverse affects or impact to safety. Furthermore, skin fasteners at an exposed surface are most susceptible to a direct strike. Accordingly, composite structure aircraft must have some protection, especially at exposed skin fasteners, fuel and hydraulic couplings. However, it is also important that this protection is economically feasible, in its initial application, in its effectiveness for minimizing resulting damage and, in subsequent consequent repair or replacement, both for continued aircraft flightworthiness and to meet economic repair targets.
Unfortunately, typical lightning strike protection approaches are complicated and difficult to implement in CFRP. One approach involves selectively integrating metal (e.g., copper foil) into or onto the composite laminate at the fastening areas. Moreover, this new approach has been expensive; is often difficult to implement/rework with labor intensive application processes both pre and post-assembly; and, has not consistently exhibited acceptable EME protection. Copper foils, for example, have been subject to wrinkling during lay-up/cure. Drilling the laminate for fastener installation may contaminate the fuel tank with copper. Even with this additional protection, in the absence of other supporting protection (e.g., fastener collar isolation, fillet/cap sealing), the structure may still have a low sparking threshold. In addition to added complexity, integrating a conductive surface protection layer into the composite wing skin may carry with it an unacceptable weight penalty.
Thus, there is a need for effective lightning protection for composite structures that is lightweight, relatively low-cost, as well as simple to apply and repair, and especially for such lightning protection for composite aircraft.